This invention relates to a magnetic recording medium, and more specifically to such medium with an antistatic property improved by use of a binder containing at least one type of fine metal particles, of either nickel or copper, and to a method of manufacturing the same.
With the growth of their market, magnetic tapes as a magnetic recording medium have been required to attain higher quality and better characteristics. One of the inherent properties of the tapes to be improved is their tendency of being statically charged troublingly while they are running. It is a phenomenon of electric charge accumulation on tape during its movement. The charge accumulated to excess can lead to an atmospheric discharge between the tape and the guides or the head. If not so serious, the discharge noise at the time of recording or playback will constitute an uncomfortable noise to the listener, adversely affecting the tone quality of the recorded or reproduced sound. It is often the case with such an easily charged tape, especially of the thin type, that the tape portion being paid off is attracted by static adhesion to the underlying base face and, out of balance with the given tension of the tape, the tape speed becomes ununiform, causing wow and flutter in an audio tape or jitter in a video tape and thus giving unsatisfactory sound or picture quality.
When the relative speed between the tape and the head is high as, for example, at the time of high speed recording, the static charge accumulation sometimes causes the tape inadequately wound on a reel to shift edgewise out of register with, or even deflect away from, the head, especially during the course of recording. This can result in deformation, creasing, or folding of the tape or, in extreme cases, the tape gets twisted round the head or other parts and is broken.
Also, in the manufacture of the magnetic tape itself, the electrostatic difficulties in the process of application and drying of the magnetic coating material, surface finishing, and slitting bring defects in the magnetic tape material.
Attempts thus far made to overcome those difficulties have included the following:
(1) Addition of electrically conductive carbon black to the magnetic coating, in an amount from 20 to 30 wt% of the magnetic material.
(2) Addition of an antistatic agent consisting of a surface active agent to the magnetic coating.
(3) Addition of fine, hard solid particles, of the order of a micron in size, to the magnetic coating material in the process of dispersion, whereby fine solids resulting from the abrasion of steel balls as a medium of the dispersion machine are mixed into the magnetic coating.
(4) Addition of a metal salt, such as iron chloride or iron nitrate, to the magnetic coating material.
These methods improve the conditions that arise from the phenomenon of static charge accumulation but sacrifice the other characteristics of the tape. Thus, there has been no way of preventing the troubles concomitant of the phenomenon without impairing the tape characteristics.
In an effort to solve these problems, the applicant previously proposed, in copending Japanese Patent Application No. 964/78, the addition of at least one type of fine metal particles, either of nickel or copper, to a magnetic powder so as to prevent troubles affecting the tape characteristics and to avoid static charge accumulation. The cited patent application discloses a magnetic recording medium made by coating a base material with a mixture of a magnetic powder and a binder which contains at least one type of fine metal particles, either of nickel or cobalt. Definite methods which may be used in preparing the fine particles of Ni or Cu to be used for that purpose are, for example:
(a) Reduction of nickel sulfate with NaBH.sub.4 or the like in an aqueous solution.
(b) Vacuum evaporation of bulk Ni lumps in a thin inert gas atmosphere of argon.
(c) Preparation of oxalate or formate of NiSO.sub.4 and reduction of the salt with a reducing gas, such as hydrogen, at elevated temperature, followed by grinding of the reduction product in a ball mill.
(d) Dissolution of a nickel carbonyl compound into a solution of a polymer in a nonaqueous solvent, followed by thermal decomposition of the resulting solution.
(e) Preparation of a fine powder by spark discharge of metal pellets in light oil.
(f) Reduction of NiSO.sub.4 or other Ni salt in a solution with a reducing agent, such as hydrazine hydrate or formalin.
The fine metal particles formed in those ways, when employed as antistatic agents for magnetic recording media, gave good results when adequate consideration was given to the usage. In manufacturing magnetic recording tapes using such antistatic agents of fine metal particles, each magnetic coating material to be applied on the base was prepared by mixing such an antistatic agent with a magnetic powder, a polymeric binder, and a solvent and then thoroughly mixing the resulting composition for dispersion in a ball mill for about 60 hours.
The metal particles obtained in accordance with the methods (a), (b), and (d) were superfine, even less than 500 A in size, and when added even in small amounts they dispersed thoroughly in magnetic recording media and performed excellently as antistatic agents. However, those methods are not suited for quantity production. The reducing agent such as NaBH.sub.4 used in the method (a) is expensive, and the vacuum evaporation (b) does not lend itself to mass production because of difficulties in connecting the process steps for continuous operation and in taking out the product powder. The method (d) of thermally decomposing a nickel carbonyl compound in a nonaqueous solution involves intricate process steps and calls for extra time and labor for the separation of the resulting powder. The finer the metal particles, the better the antistatic ability of the powder will be, but the more limitations each of the methods will have in respect of the process for quantity production and in the material aspect. Even though the product serves excellently as an antistatic agent, the cost factor will be a hindrance to its use on an industrial basis. The metal powder obtained in accordance with the method (e) is relatively large in size, in the proximity of 1000 A, and is again unsuitable for mass production. The method (c) is adapted for largescale production but the powder must be much finer in size if it is to act satisfactorily as an antistatic agent. The method (f) also requires expensive agents.
The methods (a) through (f) for preparing fine metal particles are examples of the methods taught in the above-mentioned patent application. In addition to those, some other techniques for obtaining fine metal powder have already been established in the art. After all, it appears commercially advantageous to prepare metal particles first by a method already in practice for quantity production on the industrial basis or which lends itself easily to mass production, and then switch over to a method whereby the particles are ground to a desired size, typically in the range from 1000 down to 500 A, and are uniformly distributed in the magnetic coating. The manufacturing cost will be far less than that by any single method of the prior art.
Methods suited for quantity production of the fine particles of copper and nickel include, in addition to the afore-mentioned method (c) that depends on hydrogen reduction of an organometallic salt, a method of thermally decomposing nickel carbonyl gas, a method of recovering a dendritic metal powder deposited on plates, shotting, spraying, and electrochemical deposition of Ni or Cu particles by the addition of Al or the like to a solution of a Ni or Cu salt.
After careful investigations through microscopic observation of the shape of the particles obtained in accordance with those methods, I found that the particles are mostly those retaining the skeletal structure of the mother salt and those of a secondary particle structure formed by aggregation of the primary particles. It then occurred to me that such relatively coarse particles can be pulverized, by subjection to shearing forces, to finer particles typically ranging in size from 1000 down to 500 A. Actually, mere application of shearing forces to relatively large particles will not produce a desired pulverization effect but rather invite a blocking phenomenon to a disadvantage. For attaining the desired effect, use of a medium capable of dispersing the finely divided solids is essential. As a result of an extensive research, it has now been found that, when a mixture of such a relatively coarse starting powder with a suitable amount of a high molecular or polymeric material for paint coating use is processed on a pulverizing-dispersing apparatus capable of exercising strong shearing forces, e.g., a hot two-roll mill, pressure kneader, Banbury mixer, or a combination of a kneader and a three-roll mill, a metal fines dispersed paint polymeric material for coating will result, in which fine metal particles of a desired size, formed by pulverizing the skeletal structure of the mother salt or by pulverizing the aggregated structure, are dispersed. The paint polymeric material thus obtained is in a state such that the fine particles of the primary particle size are individually wrapped by the paint material and are throughly dispersed therein. This metalfines-dispersed paint polymeric material can be dissolved and dispersed in a solvent of great dissolving power, such as methyl ethyl ketone or cyclohexane, and then mixed into a binder for use in the ordinary process for producing a magnetic recording medium. Thus, from its nature, this fines-dispersed polymeric material may be called an antistatic additive or binder.
Examples of the paint polymeric materials are vinyl chloride-vinyl acetate-vinyl alcohol copolymers (e.g., that which is manufactured under the trade designation "VAGH" by Union Carbide Corp.), polyvinyl butyral resins (e.g., "VYXL" by Union Carbide Corp.), expoxy resins (e.g., "Epiclon H-350"), vinyl chloride-vinyl acetate copolymers ("VYHH"), vinyl chloride-vinyl acetate-vinyl propionate copolymers, polybutadiene resins, butadiene-acrylonitrile copolymer resins, vinyl acetate-ethylene copolymer resins, phenoxy resins, linear saturated polyester resins (e.g., "Vylon V-300" and "Vylon V-200" by Toyobo Co.), and polyurethane resins (e.g., "Estane-5701, -5703, and -5707" by B. F. Goodrich Chemical Co. and "Nipporan 3022 and 5032" by Nippon Polyurethane Co.).
Importance is given to the compounding ratio of the fine metal particles and the polymeric material in an antistatic binder. If the proportion of the metal powder is less than 50% by weight, the shearing forces will not effectively propagate to the powder, failing to create an adequate pulverizing action. Conversely if the metal powder accounts for more than 95% of the total weight of the binder, insufficiency of the dispersion medium to effect the solids dispersion will rather cause blocking of the metal particles. For these reasons the proportion of the metal powder is desired to be between 50 and 95% of the total binder weight.
The advantages derivable from the use of the antistatic binder developed under this invention are as follows:
(1) Reduction of the cost is made possible by use of the coarse metal particles being mass produced on the industrial basis.
(2) Since the fine particles of Ni or Cu are ground to the size of primary particles and dispersed in the paint polymeric material, the properties of the binder as an antistatic agent are improved. Moreover, because a small addition is enough for that purpose, the electromagnetic conversion characteristic of the magnetic recording medium itself is not sacrificed.
(3) The fine metal particles, already dispersed as wrapped by the paint polymeric material, need no redispersion as by employing a ball mill. Furthermore, mere dissolution of the antistatic binder in an organic solvent permits easier dispersion of the fine metal particles contributing to great efficiency for the process of producing a magnetic recording medium.
(4) Being dispersedly wrapped in the polymeric material, the fine metal particles are protected from catching fire due to oxidation.
The magnetic recording medium made with use of this antistatic binder differs from the conventional Ni- or Cu-containing magnetic recording media in that the antistatic fine particles of Ni or Cu metal are uniformly dispersed in the magnetic coating layer in the form of minute particles formed by pulverization to the degree that the skeletal structure of the mother salt is no longer retained, or by disintegration of the aggregated structure.